Compact head-mounted display system protected by a hyperfine structure

ABSTRACT

There is provided an optical system, including a light-transmitting substrate ( 20 ) having at least two external major surfaces and edges, an optical element for coupling light waves into the substrate ( 20 ) by internal reflection, at least one partially reflecting surface located in the substrate ( 20 ), for coupling light waves out of the substrate ( 20 ), at least one transparent air gap film ( 110 ) including a base ( 112 ) and a hyperfine structure ( 111 ) defining a relief formation, constructed on the base, wherein the air gap film is attached to one of the major surfaces of the substrate ( 20 ), with the relief formation facing the substrate ( 20 ) defining an interface plane ( 58 ), so that the light waves coupled inside the substrate ( 20 ) are substantially totally reflected from the interface plane ( 58 ).

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, and particularly to devices which include a plurality of reflecting surfaces carried by a common light-transmissive substrate, also referred to as a light-guide element.

BACKGROUND OF THE INVENTION

One important application for compact optical elements is in head-mounted displays (HMDs), wherein an optical module serves both as an imaging lens and a combiner, wherein a two-dimensional image source is imaged to infinity and reflected into the eye of an observer. The display source can be obtained directly from either a spatial light modulator (SLM) such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), a scanning source or similar devices, or indirectly, by means of a relay lens or an optical fiber bundle. The display source comprises an array of elements (pixels) imaged to infinity by a collimating lens and is transmitted into the eye of the viewer by means of a reflecting or partially reflecting surface acting as a combiner for non-see-through and see-through applications, respectively. Typically, a conventional, free-space optical module is used for these purposes. As the desired field-of-view (FOV) of the system increases, however, such a conventional optical module becomes larger, heavier and bulkier, and therefore, even for a moderate performance device, such as a system, is impractical. This is a major drawback for all kinds of displays and especially in head-mounted applications, wherein the system should necessarily be as light and as compact as possible.

The strive for compactness has led to several different complex optical solutions, all of which on the one hand, are still not sufficiently compact for most practical applications, and on the other hand, suffer major drawbacks in terms of manufacturability. Furthermore, the eye-motion-box (EMB) of the optical viewing angles resulting from these designs is usually very small, typically less than 8 mm Hence, the performance of the optical system is very sensitive, even for small movements of the optical system relative to the eye of the viewer, and does not allow sufficient pupil motion for comfortable reading of text from such displays.

The teachings included in Publication Nos. WO 01/95027, WO 03/081320, WO 2005/024485, WO 2005/024491, WO 2005/024969, WO 2005/124427,

WO 2006/013565, WO 2006/085309, WO 2006/085310, WO 2006/087709, WO 2007/054928, WO 2007/093983, WO 2008/023367, WO 2008/129539, WO 2008/149339, WO 2013/175465 and IL 232197, all in the name of Applicant, are herein incorporated by reference.

DISCLOSURE OF THE INVENTION

The present invention facilitates the exploitation of a very compact light-guide optical element (LOE) for, amongst other applications, HMDs. The invention allows relatively wide FOVs together with relatively large EMB values. The resulting optical system offers a large, high-quality image, which also accommodates large movements of the eye. The optical system offered by the present invention is particularly advantageous because it is substantially more compact than state-of-the-art implementations, and yet it can be readily incorporated, even into optical systems having specialized configurations.

A broad object of the present invention is therefore to alleviate the drawbacks of prior art compact optical display devices and to provide other optical components and systems having improved performance, according to specific requirements.

The invention can be implemented to advantage in a large number of imaging applications, such as portable DVDs, cellular phones, mobile TV receivers, video games, portable media players or any other mobile display devices.

The main physical principle of the LOE's operation is that light waves are trapped inside the substrate by total internal reflections from the external surfaces of the LOE. However, there are situations wherein it is required to attach another optical element to at least one of the external surfaces. In that case, it is essential to confirm that on the one hand, the reflection of light waves from the external surfaces will not be degraded by this attachment, and on the other hand, that the coupling-out and the coupling-in mechanisms of the light waves from and to the LOE will not be disturbed. As a result, it is required to add at the external surfaces an angular sensitive reflective mechanism that will substantially reflect the entire light waves which are coupled inside the LOE and impinge on the surfaces at oblique angles, and substantially transmit the light waves which impinge on the surfaces close to a normal incidence.

In previous inventions (e.g., described in Publication WO 2005/024491), a reflective mechanism, wherein an angular sensitive thin film dielectric coating is applied to the surfaces of the LOE, has been illustrated. According to the present invention, an alternative reflective mechanism that utilizes an air gap film, which comprises a moth-eye structure, is presented. Moths' eyes have an unusual property: their surfaces are covered with a natural nanostructured film which eliminates reflections. This allows the moth to see well in the dark, without reflections, which give its location away to predators. The structure consists of a hexagonal pattern of bumps, each roughly 200 nm high and their centers are spaced apart about 300 nm. This kind of anti-reflective coating works because the bumps are smaller than the wavelength of visible light, so the light “sees” the surface as having a continuous refractive index gradient between the air and the medium, which decreases reflection by effectively removing the air-lens interface. Practical anti-reflective films have been made by humans using this effect, being a form of bio-mimicry. Moth eye replicas show that reflectance for normally incident light is virtually completely eliminated for these structures. Optical modeling and experiments with other shapes and dimensions of such dense uneven hyperfine periodic structures prove that it is possible to suppress reflection in wider wavelength range (from UV to IR) and wider light incidence angles (0±60 degrees).

According to the present invention the moth-eye film, or any similar hyperfine structure, is not utilized as anti-reflection film. Instead, the special hyperfine structure is exploited as the required angular sensitive reflective mechanism. When it is required to attach an optical element to the external surface of the LOE, an air gap film is cemented to the optical element such that the hyperfine structure faces the LOE after the attachment. Therefore, when the coupled-in light waves inside the LOE impinge on the hyperfine structure at different oblique angles, they “see” only the external part of the periodic structure. The actual refractive index, which is “seen” by the incoming optical light waves is, therefore, close to the refractive index of the air, and the total internal reflection mechanism is preserved. On the other hand, the air gap film is substantially transparent to the incoming light waves from the external scene or to the light waves which are coupled out from the LOE.

The invention therefore provides an optical system, including a light-transmitting substrate having at least two external major surfaces and edges, an optical element for coupling light waves into the substrate by internal reflection, at least one partially reflecting surface located in the substrate, for coupling light waves out of the substrate, at least one transparent air gap film including a base and a hyperfine structure defining a relief formation, constructed on the base, wherein the air gap film is attached to one of the major surfaces of the substrate, with the relief formation facing the substrate defining an interface plane, so that the light waves coupled inside the substrate are substantially totally reflected from the interface plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood.

With specific reference to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings are to serve as direction to those skilled in the art as to how the several forms of the invention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of an exemplary, prior art LOE;

FIG. 2 is a schematic diagram illustrating a prior art optical device for collimating input light-waves from a display light source;

FIG. 3 is a schematic diagram illustrating a prior art system for collimating and coupling-in input light-waves from a display light source into an LOE;

FIG. 4 is a schematic diagram illustrating another prior art system for collimating and coupling-in input light-waves from a display light source into a substrate, wherein the collimating module is attached to the substrate;

FIG. 5 illustrates an exemplary embodiment of the present invention, wherein a negative lens is attached to an external surface of the light-guide optical element, in accordance with the present invention;

FIG. 6 illustrates an exemplary embodiment of the present invention, wherein negative and positive lenses are attached to the external surfaces of the light-guide optical element, in accordance with the present invention;

FIGS. 7a and 7b are two- and three-dimensional schematic views of an exemplary embodiment of an air gap film, wherein a hyper-fine periodic structure of transparent dielectric material arranged at a small pitch shorter than the wavelengths of the photopic region, is constructed on a flat transparent substrate;

FIGS. 8a and 8b respectively illustrate a side view and a top view of an exemplary air gap film;

FIGS. 9a and 9b respectively illustrate a side view and a top view of an exemplary air gap film for an internal cross section which is close to the base;

FIGS. 10a and 10b respectively illustrate a side view and a top view of an exemplary air gap film for an external cross section which is close to the air;

FIG. 11 illustrates a side view of a light wave impinging on the upper side of a hyperfine structure at an oblique angle, in accordance with the present invention;

FIG. 12 illustrates an air-gap film which is attached to the external surface of an LOE, wherein a coupled light wave impinges on the interface surface between the LOE and the film, in accordance with the present invention;

FIGS. 13a and 13b respectively illustrate a front view of an eyeglasses system and a top view of an LOE embedded between two optical lenses and assembled inside the eyeglasses frame, in accordance with the present invention;

FIGS. 14a, 14b and 14c respectively illustrate a non-monolithic optical element comprising an LOE embedded between a front positive lens and a rear negative lens, mounted together inside a frame without adhesive, in accordance with the present invention;

FIGS. 15a, 15b and 15c respectively illustrate an alternative method for embedding an LOE between two optical lenses, utilizing a peripheral bonding technique, in accordance with the present invention;

FIGS. 16a, 16b and 16c respectively illustrate an alternative method for monolithically embedding an LOE between two optical lenses, in accordance with the present invention, and

FIGS. 17a, 17b and 17c respectively illustrate an LOE embedded between two flat substrates and assembled inside a frame, in accordance with the present invention.

FIG. 18 illustrates an exemplary embodiment of the present invention, wherein the coupling-in as well as the coupling-out elements are diffractive optical elements, and

FIG. 19 illustrates an exemplary embodiment of the present invention, wherein the optical module is embedded in a hand-carried display system.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a sectional view of a prior art optical system including a planar substrate 20 and associated components (hereinafter also referred to as an “LOE”), utilizable in the present invention. An optical means, e.g., a reflecting surface 16, is illuminated by light waves 18, which are collimated from a display of a light source (not shown). The reflecting surface 16 reflects incident light waves from the source, such that the light waves are trapped inside the planar substrate 20 of the LOE, by total internal reflection. After several reflections of the major lower and upper surfaces 26, 28 of the substrate 20, the trapped waves reach an array of selective partially reflecting surfaces 22, which couple the light out of the substrate into a pupil 25 of an eye 24 of a viewer. Herein, the input surface of the LOE will be regarded as the surface through which the input light waves enter the LOE, and the output surface of the LOE will be regarded as the surface through which the trapped waves exit the LOE. In the case of the LOE illustrated in FIG. 1, both the input and the output surfaces are on the lower surface 26. Other configurations, however, are envisioned in which the input and the image waves could be located on opposite sides of the substrate 20, or when the light is coupled into the LOE through a slanted edge of the substrate.

As illustrated in FIG. 2, the s-polarized input light-waves 2 from a display light source 4 are coupled into a collimating module 6 through its lower surface 30, which module is usually composed of a light-waves transmitting material. Following reflection-off of a polarizing beamsplitter 31, the light-waves are coupled-out of the substrate through surface 32 of the collimating module 6. The light-waves then pass through a quarter-wavelength retardation plate 34, reflected by a reflecting optical element 36, e.g., a flat mirror, return to pass again through the retardation plate 34, and re-enter the collimating module 6 through surface 32. The now p-polarized light-waves pass through the polarizing beamsplitter 31 and are coupled out of the light-guide through surface 38 of the collimating module 6. The light-waves then pass through a second quarter-wavelength retardation plate 40, collimated by a component 42, e.g., a lens, at its reflecting surface 44, return to pass again through the retardation plate 34, and re-enter the collimating module 6 through surface 38. The now s-polarized light-waves reflect off the polarizing beamsplitter 31 and exit the collimating module through the upper surface 46. The reflecting surfaces 36 and 44 can be materialized either by a metallic or a dielectric coating.

FIG. 3 illustrates how a collimating module 6, constituted by the components detailed with respect to FIG. 2, can be combined with a substrate 20, to form an optical system. The output light-waves 48 from the collimating module 6 enter the substrate 20 through its lower surface 26. The light waves entering the substrate 20 are reflected from optical element 16 and trapped in the substrate, as illustrated in FIG. 2. Now, the collimating module 6, comprising the display light source 4, the folding prisms 52 and 54, the polarizing beamsplitter 31, the retardation plates 34 and 40 and the reflecting optical elements 36 and 42, can easily be integrated into a single mechanical module and assembled independently of the substrate, even with non-accurate mechanical tolerances. In addition, the retardation plates 34 and 40 and the reflecting optical elements 36 and 42 could be cemented together, respectively, to form single elements.

It would be advantageous to attach all the various components of the collimating module 6 to the substrate 20, to form a single compact element resulting in a simplified mechanical module. FIG. 4 illustrates such a module, wherein the upper surface 46 of the collimating module 6 is attached at the interface plane 58, to the lower surface 26 of the substrate 20. The main problem of this configuration is that the attaching procedure cancels the previously existing air gap 50 (illustrated in FIG. 3) between the substrate 20 and the collimating module 6. This air gap is essential for trapping the input light waves 48 inside the substrate 20. The trapped light waves 48 should be reflected at points 62 and 64 of the interface plane 58. Therefore, a reflecting mechanism should be applied at this plane, either at the major surface 26 of the substrate 20, or at the upper surface 46 of the collimating module 6. A simple reflecting coating cannot, however, be easily applied, since these surfaces should also be transparent to the light waves that enter and exit the substrate 20 at the exemplary points 66. The light waves should pass through plane 48 at small incident angles, and reflect at higher incident angles. Usually, the passing incident angles are between 0° and 15° and the reflecting incident angles are between 40° and 80°.

In the above-described embodiments of the present invention, the image which is coupled into the LOE is collimated to infinity. There are applications, however, where the transmitted image should be focused to a closer distance, for example, for people who suffer from myopia and cannot properly see images located at long distances. FIG. 5 illustrates an optical system utilizing a lens, according to the present invention. An image 80 from infinity is coupled into a substrate 20 by a reflecting surface 16, and then reflected by an array of partially reflective surfaces 22 into the eye 24 of the viewer. The (plano-concave) lens 82 focuses the images to a convenient distance and optionally corrects other aberrations of the viewer's eye, including astigmatism. The lens 82 can be attached to the surface of the substrate at its flat surface 84. As explained above with regard to FIG. 4, a thin air gap must be preserved between the lens and the substrate, to ensure the trapping of the image light waves 80 inside the substrate by total internal reflection.

In addition, in most of the applications related to the present invention, it is assumed that the external scene is located at infinity; however, there are professional or medical applications where the external scene is located at closer distances. FIG. 6 illustrates an optical system for implementing a dual lens configuration, based on the present invention. Image light waves 80 from infinity are coupled into a substrate 20 by a reflecting surface 16 and then reflected by an array of partially reflective surfaces 22 into the eye 24 of the viewer. Another image 86 from a close distance scene is collimated to infinity by a lens 88 and then passed through the substrate 20 into the eye 24 of a viewer. The lens 82 focuses images 80 and 86 to a convenient distance, usually (but not necessarily always) the original distance of the external scene image, and corrects other aberrations of the viewer's eye, if required.

The lenses 82 and 88 shown in FIGS. 5 and 6 are simple plano-concave and plano-convex lenses, respectively, however, to keep the planar shape of the substrate, it is possible instead to utilize Fresnel lenses, which can be made of thin molded plastic plates with fine steps. Moreover, an alternative way to materialize the lenses 82 or 88, instead of utilizing fixed lenses as described above, is to use electronically controlled dynamic lenses. There are applications where the user will not only be able to see a non-collimated image but also to dynamically control the focus of the image. It has been shown that a high resolution, spatial light modulator (SLM) can be used to form a holographic element. Presently, the most popular sources for that purpose are LCD devices, but other dynamic SLM devices can be used as well. High resolution, dynamic lenses having several hundred lines/mm are known. This kind of electro-optically controlled lenses can be used as the desired dynamic elements in the present invention, instead of the fixed lenses described above in conjunction with FIGS. 5 and 6. Therefore, in real time, a user can determine and set the exact focal planes of both the virtual image projected by the substrate and the real image of the external view.

As illustrated in FIG. 6, it would be advantageous to attach the lenses 82 and 88 to the substrate 20, to form a single, compact simplified mechanical module. Clearly, the main problem as hereinbefore described, is that the attaching procedure cancels the previously existing air gap between the substrate 20 and the lenses 82 and 88, which gaps are essential for trapping image light waves 80 inside the substrate 20. The trapped image light waves 80 should be reflected at point 90 of the interface plane 84 and transmitted through the same plane at point 92. Therefore, a similar partially reflecting mechanism as described above in relation to FIG. 4 should be applied at this plane.

To achieve the required partially reflecting mechanism, it is possible to apply an angular sensitive thin film coating at the major surfaces of the substrate; however, the fabrication of this embodiment can be complicated and expensive. An alternative way for realizing the required partially reflecting mechanism is to attach a transparent air gap film 110 to the major surfaces of the substrate, as illustrated in FIGS. 7a and 7b . The term air gap film relates to an optical device which has on its surface a hyper-fine periodic structure 111 of transparent dielectric material arranged at a small pitch shorter than the wavelengths of the photopic region, e.g., an optical device such as moth-eye film having a dense (uneven) hyperfine periodic structure 111 (hereinafter referred to as “relief formation”), which is constructed on a flat transparent substrate 112 (hereinafter referred to as “base” 112 or “base film” 112). The height of the relief formation should preferably (but not necessarily always) be less than 1 micron.

As seen in FIGS. 8a, and 8b , any cross section 121 parallel to the surface of the air gap film 110 has a periodic formation, wherein the proportional portion of the dielectric material 123 in the relief formation is changed gradually as a function from the film itself.

As further seen in FIGS. 9a, 9b and 10a and 10b , in the internal cross section 124, which is close to the base film 112, i.e., the lower portion of the hyperfine structure 111, the proportional portion of the dielectric material 125 in the relief formation 126 is maximal and substantially close to 1, while in the external cross section 127, i.e., close to the upper portion of the hyperfine structure 111, the proportional portion of the dielectric material 128 in the relief formation 129 is minimal, namely, significantly lower than in material 125, and substantially equal to zero.

Typically, when light waves pass through an optical device having a periodic structure, diffraction of light occurs and the brightness of the zero order of the diffracted light, namely, the light which is transmitted through the device without any diffraction, is considerably reduced. When the pitch of the hyper-fine periodic structure is, however, considerably shorter than the wavelength of the incoming light waves, no diffraction occurs. Instead, since the optical waves “see” a medium having a refractive index which is the average of the materials contained in this medium, effective anti-reflection properties can be obtained.

On the other hand, as illustrated in FIG. 11, when the light waves 130 impinge on the periodic hyperfine structure 111 at the upper side of the structure at oblique angles, they “see” only the external part of the periodic structure, wherein the proportional part of the transparent material is very low. Therefore, the actual refractive index, which is “seen” by the incoming optical waves, is close to the refractive index of the air 131.

As a result, and as illustrated in FIG. 12, when such an air-gap film is attached to the external surface 28 of the substrate 20, the coupled light waves 130 impinge on the interface surface 132 between the substrate and the film at angles higher than the critical angle, the air 131 confined between the film and the substrate provides an optical isolation due to the air-like refractive index in the boundary surface. Therefore, the phenomena of total internal reflection of the coupled-in light waves from the external surface will be preserved and the light waves will be contained inside the substrate.

The geometrical characteristic of the hyperfine structure, such as the height, peak-to-peak and width thereof, can usually be between 10 to 800 nanometers. In addition, the exact shape and of the hyperfine structure should not necessarily be that of the moth eye. Any other nano-structure shape, such as pyramids, prisms, cones and others, can be utilized. Moreover, the hyperfine structure should not necessarily be specifically periodic, although a periodic structure is usually easier to fabricate. This hyperfine structure, however, should fulfill the following requirements: on one hand, the structure should be solid enough not to collapse during the attaching process and, on the other hand, the proportional portion of the dielectric material in the external cross-section of the structure, should be substantially equal to zero, to maintain the total internal reflection phenomena inside the substrate. In addition, the size of the basic elements of the hyperfine structure should not be too large, in order to avoid diffraction effects. Reducing the thickness of the hyperfine structure to below 100 nm, however, might undesirably allow the penetration of the trapped waves through the air gap film and the deterioration of the total internal reflection phenomena. As a result, a typical required value for the hyperfine structure thickness is between 200 and 300 nm.

FIG. 13a illustrates a front view of an eyeglasses system 140 and FIG. 13b a top view of a substrate 20 which is embedded between two optical lenses 141, 142 and assembled inside the eyeglasses frame 143. As seen, in addition to the optical elements, the frame can contain other accessories including a camera 144, a microphone 145, earphones 146, USB connectors, memory cards, an inertial measurement unit (IMU), and the like.

FIGS. 14a, 14b and 14c illustrate a non-monolithic optical element 150 comprising a substrate 20 embedded between front positive lens 151 and rear negative lens 152, mounted together inside a frame 154 without adhesive. Air gap films 110 (FIG. 14c ) are placed or bonded between the substrate 20 and the lenses 151, 152, wherein the hyperfine structures 111, respectively face the external surfaces 26 and 28 of the substrate 20. The air gap films 110 can be directly cemented on the planar surfaces of the optical lenses 151 and 152 using pressure-sensitive adhesive (PSA), or can be fabricated directly as an integral part of the lenses utilizing embossing, injection molding, casting, machining, soft lithography or any other direct fabrication method. The embedded optical element 150 can be assembled inside the frame 154 utilizing pressure or cementing techniques.

An alternative method for monolithically embedding the substrate 20 between the two optical lenses is illustrated in FIGS. 15a, 15b and 15c . The substrate 20 is embedded between the optical lenses utilizing a peripheral bonding technique. The front lens 151 and rear lens 152 are cemented to the peripheral edges of the substrate 20 using non-optical adhesive or any other high-viscosity adhesive 156 that mount all components together. The viscosity of the adhesive should be high enough in order to prevent leakage of the adhesive into the air pockets 131, which are confined between the film 110 and the substrate 20. Such a leakage can eliminate the air gap which is required to preserve the total internal reflection of the light waves from the external surfaces of the substrate. The required adhesive 156 can, for example, be OP-67-LS or any room temperature vulcanization (RTV) silicone.

Another alternative method for monolithically embedding the substrate 20 between the two optical lenses is illustrated in FIGS. 16a, 16b and 16c . The production procedure of the embedded element is as follows: placing the air gap film 110, with the hyperfine structures 111 facing the external surfaces 26 and 28 of the substrate 20; utilizing attaching techniques such as static electricity; preparing a mold 160 having the required external shape of the element; inserting the substrate 20 into the mold; casting or injecting the polymer into the mold, curing the polymer by UV or by changing the polymer temperature, and finally, ejecting the embedded element from the mold. As explained above in relation to FIGS. 15a to 15c , it is also important that the hyperfine regions will be isolated from the injected material during the injection molding process, in order to prevent a leakage of the material into the air pockets 131 between the substrate 20 and the air gap film 110.

FIGS. 13a to 16c illustrate different methods for forming an optical component comprising a substrate embedded between two optical lenses, however, there are embodiments wherein it is required to attach planar elements to the external surfaces of the substrate. An example for such an embodiment is illustrated in FIG. 4, wherein the collimating element 6 is attached to the substrate 20. Other reasons for attaching a flat element to a substrate can be for mechanically protecting the substrate to enhance the user's eye-safety, or applying a coating on the external surface of the flat element to achieve various characteristics such as, photochromic response, scratch resistance, super-hydrophobicity, tinted (colored) view, polarization, anti-finger print, and the like.

A substrate 20 embedded between two flat substrates 162 and 164 and assembled inside frames 166, 167 is illustrated in FIGS. 17a, 17b and 17c . The embedding process of the substrate and the flat substrates 20 can be materialized utilizing mechanical attachment, peripheral cementing or monolithic fabrication. Embedding processes can include attaching only a single element to one of the external surfaces of the substrate or combining different elements, such as flat substrates as well as curved lenses.

In all the embodiments illustrated so far the element for coupling light waves out of the substrate is at least one flat partially reflecting surface located in said substrate, which is usually coated with a partially reflecting dielectric coating and is non-parallel to the major surfaces of said substrate. However, the special reflective mechanism according to the present invention can be exploited also for other coupling-out technologies. FIG. 18 illustrates a substrate 20, wherein the coupling-in element 170 or the coupling-out element 172 are diffractive elements. In addition, other coupling-out elements, such as a curved partially reflecting surface, and other means, can be used.

The embodiments of FIGS. 13-17 are just examples illustrating the simple implementation of the present invention. Since the substrate-guided optical element, constituting the core of the system, is very compact and lightweight, it could be installed in a vast variety of arrangements. Many other embodiments are also possible, including a visor, a folding display, a monocle, and many more. This embodiment is designated for applications where the display should be near-to-eye; head-mounted, head-worn or head-carried. There are, however, applications where the display is located differently. An example of such an application is a hand-carried device for mobile application, such as, for example, a smartphone or smartwatch. The main problem of these smart devices is the contradiction between the required small size and volume and the desired high quality image.

FIG. 19 illustrates an alternative method, based on the present invention, which eliminates the current necessary compromise between the small size of mobile devices and the desire to view digital content on a full format display. This application is a hand-held display (HHD) which resolves the previously opposing requirements, of achieving small mobile devices, and the desire to view digital content on a full format display, by projecting high quality images directly into the eye of the user. An optical module including the display source 4, the folding and collimating optics 190 and the substrate 20 is integrated into the body of a smart device 210, where the substrate 20 replaces the existing protective cover-window of the phone. Specifically, the volume of the support components, including source 4 and optics 190, is sufficiently small to fit inside the acceptable volume for modern smart device. In order to view the full screen, transmitted by the device, the window of the device is positioned in front of the user's eye 24, observing the image with high FOV, a large eye-motion-box and a comfortable eye-relief. It is also possible to view the entire FOV at a larger eye-relief by tilting the device to display different portions of the image. Furthermore, since the optical module can operate in see-through configuration, a dual operation of the device is possible; namely there is an option to maintain the conventional display 212 intact. In this manner, the standard display can be viewed through the substrate 20 when the display source 4 is shut-off. In a second, virtual-mode, designated for massive internet surfing, or high quality video operations, the conventional display 212 is shut-off, while the display source 4 projects the required wide FOV image into the eye of the viewer through the substrate 20. Usually, in most of the hand-carried smart devices, the user can operate the smart device by using a touchscreen which is embedded on the front window of the device. As illustrated in FIG. 19, the touchscreen 220 can be attached to a smart device by directly cementing it on the external surface air gap films 110, which is located on the substrate 20. 

What is claimed is:
 1. An optical system, comprising: a light-transmitting substrate having at least two external major surfaces and edges; an optical element for coupling light waves into the substrate by internal reflection; at least one partially reflecting surface located in the substrate, for coupling light waves out of the substrate; at least one transparent air gap film including a base and a hyperfine structure defining a relief formation, constructed on the base; wherein the air gap film is attached to one of the major surfaces of the substrate, with the relief formation facing the substrate defining an interface plane, so that the light waves coupled inside the substrate are substantially totally reflected from the interface plane.
 2. The optical system according to claim 1, wherein the air gap film forms air pockets between the relief formation and the substrate.
 3. The optical system according to claim 1, wherein the relief formation includes dielectric material.
 4. The optical system according to claim 1, wherein a proportional portion of the relief formation at a cross section parallel to the base, gradually changes as a function of the distance of the cross section from the base.
 5. The optical system according to claim 3, wherein a proportional portion of the dielectric material of the relief formation at an internal cross section close to the base, is maximal.
 6. The optical system according to claim 3, wherein a proportional portion of the dielectric material of the relief formation at an internal cross section close to the base, is substantially equal to one.
 7. The optical system according to claim 3, wherein a proportional portion of the dielectric material of the relief formation at an external cross section close to the interface plane, is minimal.
 8. The optical system according to claim 7, wherein a proportional portion of the dielectric material of the relief formation at an external cross section close to the interface plane, is substantially equal to zero.
 9. The optical system according to claim 1, wherein a refractive index of the relief formation at an external cross section close to the interface plane, is substantially equal to a refractive index of air.
 10. The optical system according to claim 1, wherein the relief formation is a periodic relief formation.
 11. The optical system according to claim 1, wherein the relief formation has a configuration of a moth eye.
 12. The optical system according to claim 1, wherein the relief formation is selected from the group of arrays of prisms, cones and pyramids.
 13. The optical system according to claim 1, wherein the height of the relief formation is less than 1 micron.
 14. The optical system according to claim 1, further comprising at least one optical element having at least one flat external surface facing the substrate, wherein the base is optically cemented to the external surface of the element and the relief formation facing the substrate.
 15. The optical system according to claim 14, wherein the optical element is a flat transparent substrate.
 16. The optical system according to claim 14, wherein the optical element is a collimating module.
 17. The optical system according to claim 14, wherein the optical element is a lens.
 18. The optical system according to claim 1, further comprising a second air gap film attached to a second of the major surfaces of the transparent substrate, with the relief formation facing the substrate surface.
 19. The optical system according to claim 18, further comprising two lenses having each at least one flat surface, wherein the bases of the first and second air gap films are cemented to the flat surfaces of the lenses.
 20. The optical system according to claim 14, wherein the substrate and the optical element are assembled inside an eyeglasses frame.
 21. The optical system according to claim 20, wherein the substrate and the optical element are mounted together inside the frame without adhesive.
 22. The optical system according to claim 14, wherein the substrate and the optical element are embedded together utilizing a peripheral bonding technique.
 23. The optical system according to claim 14, wherein the substrate and the optical element are monolithically embedded together.
 24. The optical system according to claim 1, wherein the partially reflecting surface for coupling light waves out of the substrate, is a flat surface.
 25. The optical system according to claim 24, wherein the partially reflecting surface for coupling light waves out of the substrate, is coated with a partially reflecting dielectric coating.
 26. The optical system according to claim 24, wherein the partially reflecting surface for coupling light waves out of the substrate, is non-parallel to the major surfaces of said substrate.
 27. The optical system according to claim 1, wherein the optical element for coupling light waves into the substrate by internal reflection, is a diffractive element.
 28. The optical system according to claim 1, wherein the partially reflecting surface for coupling light waves out of the substrate, is a diffractive element.
 29. The optical system according to claim 1, wherein the partially reflecting surface for coupling light waves out of the substrate, is a curved surface.
 30. The optical system according to claim 14, wherein the substrate and the optical element are assembled inside a smart device.
 31. The optical system according to claim 30, wherein the smart device is a smartphone.
 32. The optical system according to claim 30, wherein the smart device is a smartwatch.
 33. The optical system according to claim 30, wherein the optical element is a touchscreen. 